(1) Field of the Invention
This invention relates to the preparation of thermoplastic starch-vegetable oil graft copolymers and moldable compositions containing a biofiber reinforced in the starch-vegetable oil graft copolymer matrix, which are preferably biodegradable. More specifically, the present invention relates to a process of making biofiber reinforced starch-vegetable oil graft copolymer blends which can be injection molded to give products which are totally biodegradable, easily processed and possessing good mechanical properties.
(2) Description of Related Art
Traditionally, plastics have been formulated to result in strong, light-weight, durable, and bioresistant polymeric materials. It is the durability and indestructibility that makes plastic the material of choice for many applications. However, these same properties are problems when the plastics enter the waste stream. The recent trend is to create biodegradable plastics, most of such plastics being first commercialized in the mid 1980's (Barenberg, S. A., et al., “Degradable Materials: Perspectives, Issues and opportunities” (1990: and Vert, M., et al., “Biodegradable Polymers and Plastics” (1992)).
Among the first ‘biodegradable’ plastics made were blends of non-biodegradable polyolefins with starch which were at best only partially biodegradable (Gilmore, D. F., et al., “The Fate of ‘Biodegradable’ Plastics in municipal Leaf Compost”, J. Industr. Microbiol. 10, 199-206 (1992); Barak, Y., et al., J. Environ. Qual., 20 173 (1991); Krupp, L. R., et al., Environ. Sci. Techol. 26 193 (1992); and Swanson, C. L., et al., J. Environm. Polymer Degrad. 1(2), 155-165 (1993). These plastics are not compatible with waste management infrastructures, such as composting. Moreover, at that time, the appropriate infrastructures capable of dealing with biodegradables did not exist. Instead of composting, these products generally ended up in sanitary landfills.
Landfills, in general, are a poor choice as a repository of plastic and organic waste. Landfills are plastic-lined tombs designed to retard biodegradation by providing little or no moisture with negligible microbial activity. Organic waste, such as lawn and yard waste, paper, and food waste should not be entombed in such landfills to be preserved for posterity. Accordingly, there is a growing trend to divert these materials into composting facilities, which allow them to be biodegraded to produce humus or compost. This compost can then be used as a valuable soil additive for new plant growth.
When plastics are designed to be biodegradable, utilizing renewable resources as the major raw material component, the plastics can become part of an ecologically sound solution.
Biodegradation of natural materials produces valuable compost as the major product, in addition to water and carbon dioxide. Such carbon dioxide is fixed or neutral and therefore does not contribute to an increase in the greenhouse gases.
U.S. Pat. No. 5,095,054 to Lay et al (1992) discloses the use of water as a plasticizer for starch (referred to as starch “destructurization”) in order to make the material processable in for example an extruder. Products derived therefrom tend to have the problem of rapidly losing water to the environment by evaporation. As a result, this type of material tends to become brittle with age. These materials are also highly water sensitive which is undesirable in the majority of applications of thermoplastic products. To address this issue of water sensitivity, the patent also includes various blends of destructured starch with a variety of synthetic petroleum-based plastics. Such blend compositions, along with earlier starch-filled polyolefins, are at best only biodisintegrable and not fully biodegradable. Starch-polyolefin compositions have been reported by the Fertec group (PCT Int. Pat. Appl. WO 92/14782, Bastioli, et al. “Biodegradable Compositions Based on Starch and Thermoplastic Polymers”, Novamont S. P. A., (1992); PCT Int. Pat Appl. WO 91/02025, C. Bastioli, et al., “A Polymer Composition including Destructured Starch and an Ethylene Copolymer”, Novamont S. P. A. (1991) (See also WO 91/02023 and WO 91/02024)).
U.S. Pat. No. 4,873,270 to Aime et al., (1989), describes blends of polyurethane with for example poly(vinylchloride) and a carbohydrate such as potato flour. U.S. Pat. Nos. 3,850,862 and 3,850,863 to Clendinning et al (1974), disclose blends of a naturally biodegradable product, such as tree bark, protein, starch, peat moss, saw dust, etc., with a dialkanoyl polymer (U.S. Pat. No. 3,850,862) such as poly(alkylene adipate), or with an oxyalkanoyl polymer (U.S. Pat. No. 3,850,863), such as poly(caprolactone). U.S. Pat. No. 4,863,655 to Lacourse et al (1989) discloses water-soluble high amylose starch containing poly(vinyl alcohol). This type of modified starch is highly hydrophilic and water soluble; the starch contains about 5% by weight propylene oxide corresponding to a theoretical degree of substitution of 0.19. This is a very low degree of substitution compared with the maximum degree of substitution for starch which is 3.0 according to the three available hydroxyl groups on the anhydroglucose repeat unit. The poly(vinyl alcohol) typically used as a blend component further adds to the water-sensitive nature of these materials.
As discussed earlier, prior art blend compositions are at best biodisintegrable and not fully biodegradable. In composting, the non-biodegradable components will be persistent resulting in an irreversible build-up of these components in the environment causing reduced productivity and fertility of the soil. Even if such ‘biodegradable’ blend compositions are partially biodegradable, the resulting compost will have very little value. In fact, these recalcitrant components will be present in the final compost at significantly higher concentration levels than in the original waste mixture.
Graft copolymerization of a vinyl monomer on the starch backbone was used to modify starch. Fanta and Bagley (Fanta, G. F., et al., Encyclopedia of Polymer Science, John Wiley & Sons; New York (1970: and Fanta, G. F., Block and Graft Copolymers—Vol. 1, John Wiley & Sons; New York (1973) have reviewed the synthesis and discussed some applications of starch graft copolymers. Otey et al (Otey, F. H., et al., Industrial Engineering Chemistry Products Research Development, 19, 592 (1980); Otey, F. H., et al., Industrial Engineering Chemistry Products Research Development, 23, 284 (1984); and Otey, R. P., et al., Industrial Engineering Chemistry Products Research Development, 26, 1659 (1987)) blended starch with poly(ethylene-co-acrylic acid) (EAA). In these papers, the authors suggested the formation of hydrogen bonds between the carboxylic group in EAA and the hydroxyl group in starch. Increasing the level of starch decreased the percentage elongation of the film and increased the diffusion rate of water. Similar complexes like EAA can also be formed with the hydroxyl groups of the polyethylene-vinyl alcohol (EVOH) copolymer. Bloembergen et al have reported on blends and alloys containing lignocelluloses like starch, cellulose acetate etc. U.S. Pat. No. 5,314,934 to Tomka et al provides a process to produce a polymer mixture essentially consisting of thermoplastically processable starch (up to 70-wt %). During this mixture process, an esterification reaction takes place between the maleic anhydride groups in the terpolymer with free hydroxyl groups in the starch. These reactive blends can be blown into film with properties comparable to LDPE. U.S. Pat. No. 5,234,977 to Bastioli et al discloses a material used for the product of biodegradable articles in film, sheet or fiber form, which can be produced by extrusion from a molten mass that includes a synthetic thermoplastic polymer and a destructured starch to which a boron containing compound such as boric acid has been added.
U.S. Pat. Nos. 5,618,341, 5,679,145, 5,858,824, 6,168,857 and 6,231,970 to Andersen et al explain thermoplastic starch compositions that include a particulate filler and an optional fiber. These compositions may be shaped into a wide variety of useful articles, such as sheets, films, containers and packaging materials. The overall composition in this case is typically more environmentally friendly compared to conventional thermoplastic materials.
U.S. Pat. No. 5,928,737 to Hammer et al discloses thermoplastic starch compositions formed by heating granular starch with an additive and further reinforced with fiber and protein to form sausage casings.
U.S. Pat. No. 6,521,147 to Arentsen et al provides a method and apparatus for manufacturing fiber-reinforced foamed paper-like products. Further, U.S. Pat. No. 6,565,640 to Bengs et al provides compositions based on starch and/or modified starch and comprising plasticizers which can be processed thermoplastically to give shaped articles which are biodegradable and physiologically non-hazardous. These articles may be used as packaging or casing for food or drink or pharmaceutical products, or also for the controlled release of active substances, or else for producing temporary protective coatings.
More recently, U.S. Pat. No. 6,709,526 to Bailey et al provides compositions containing starch, a polymer that is substantially compatible with starch and has a weight-average molecular weight of at least 500,000 g/mol such that the polymer forms effective entanglements or associations with neighboring starch molecules, and preferable at least one additive to improve melt flow and melt processability. The composition is especially suitable for uniaxial and biaxial extensional processes to make fibers, films, foams and like products.
The prior art on biodegradable materials and blends is restricted to starch-based materials in which the starch component is hydrophilic (water sensitive). No prior art exists on blends containing starch oil graft copolymer products and their biofiber reinforced composites as fully biodegradable products which are readily processable on conventional plastics processing equipment such as extruders injection molders, etc.
Objects
It is an object of the present invention to achieve good processability and mechanical properties with a novel starch graft co-polymer product. It is further an object to provide such processable products which are also biodegradable. It is also an object to provide new starch-based materials which utilize agricultural resources and return those resources to nature in an environmentally sound manner. These and other objects will become increasingly apparent by reference to the following description. The substance and advantages of the present invention will become increasingly apparent by reference to the following description and the drawings.